1. Field of the Invention
The present invention relates generally to diesters of tetrabromophthalic anhydride, to a method of preparing said diesters, to polyurethane foam-forming compositions containing said diesters, and to fire-retardant, self-extinguishing, polyurethane foams prepared from said compositions. More particularly, the invention relates to diesters prepared by reacting tetrabromophthalic anhydride and a polyhydroxy compound containing at least 3 hydroxyl groups. The reaction is carried out in one step by heating the tetrabromophthalic anhydride and polyhydroxy compound in the presence of an alkaline material at a temperature of from about 130.degree. C. to about 190.degree. C. The resulting diesters of tetrabromophthalic anhydride may be combined with other polyols to form blends which are useful in foam-forming compositions comprising, in addition to the polyol blend, a catalyst, a surfactant, a suitable blowing agent, and an isocyanate. These compositions are especially useful in the preparation of polyurethane foams which are both fire-retardant and self-extinguishing.
2. Description of the Prior Art
Derivatives of tetrabromophthalic anhydride, including esters thereof, have been reported in the literature. The most commonly described materials are the half-esters or monoesters prepared by reacting one mol of a compound having hydroxyl, carboxyl, or sulfhydryl groups with one mol of the anhydride.
Several diesters have also been reported. These materials have been prepared by a two-step process which comprises, first, reacting the tetrabromophthalic anhydride with one of the compounds described above to prepare the half-ester and, subsequently, reacting the half-ester with an epoxide to form the diester. Diesters prepared in this manner are disclosed, for example, in the paper entitled "Tetrabromophthalic Anhydride in Flame-Retardant Polyurethane Foams" by Pape et al. in An. SPI Tech. Conf. Tech. Papers, Vol. 26, page 695, 1968; in U.S. Pat. No. 3,455,886 issued to Versnel; and in U.S. Pat. No. 3,585,185 issued to Levis et al.
Polymeric esters have also been prepared, often in a one-step procedure, employing tetrabromophthalic anhydride and hydroxyl-containing compounds. Such materials are disclosed, for example, in U.S. Pat. No. 3,285,995 issued to Nametz et al.
However, simple diesters of tetrabromophthalic anhydride have not previously been reported. As used herein, the term simple diesters refers to products prepared in one step by reacting tetrabromophthalic anhydride and a polyhydroxy compound.
Conventional polyurethane foams, while they have excellent insulating and shock absorbing properties, have been somewhat restricted in their usefulness due to the tendency of the foams to burn especially when exposed to a direct flame. Due to this tendency to burn, these materials have frequently been used only in applications where they are protected by another material which will act as a shield and prevent the exposure of the foam itself to an open flame. A variety of additives have previously been suggested for inclusion in polyurethane foams which would make these materials fire-retardant and/or self-extinguishing.
As used herein, the term "fire-retardant polyurethane foam" refers to a foam which will not burn or support combustion in air and which has an Oxygen Index, when measured in accordance with A.S.T.M. D-2863-70, of greater than 21. Said foams also have a flame-spread index equal to 25 or less when measured in accordance with the A.S.T.M. E-84 test. Also, as used herein, the term "self-extinguishing" refers to a foam which will only burn as long as a flame is directly impinged upon it. Removal of the flame results in extinguishment of the foam.
Tetrabromophthalic anhydride and derivatives thereof have been suggested for use in polyurethane foams. However, the previously available derivatives of tetrabromophthalic anhydride could not easily be employed in the preparation of polyurethane foams. Polymeric derivatives, because of their high molecular weights and low hydroxyl numbers, result in foams having a decreased cross-link density, poor humid aging properties, and only marginal improvements in the fire-retardant and self-extinguishing properties of the foam. These materials, as well as the half-esters, are often incompatible with other components, particularly the blowing agent, employed in the foam-forming compositions and, therefore, are of only limited utility. Also, the addition of half-esters often has a deleterious effect on the physical properties of the foams including the humid aging characteristics thereof.
In accordance with the present invention, simple diesters of tetrabromophthalic anhydride are prepared. The diesters have also been found to be useful in polyol blends which may be employed in the preparation of fire-retardant, self-extinguishing, polyurethane foams without adversely affecting the physical properties of said foams.